JP2760663B2 - Control rod drive mechanism - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[Object of the Invention]

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control rod drive mechanism incorporated in a boiling water reactor.

[0003]

2. Description of the Related Art A schematic configuration of a boiling water reactor is shown in a partial sectional view of FIG. 3, in which a number of fuel assemblies are loaded in a reactor pressure vessel 1 to form a core portion 2. A large number of control rods 3 are inserted between the fuel assemblies from below at predetermined intervals. This control rod 3 is connected to a control rod drive mechanism (hereinafter, CR
The power of the reactor is controlled by inserting or withdrawing the core into or out of the reactor core part 2 by D) 4 and appropriately adjusting the set position.

The CRD 4 includes a plurality of control rod drive mechanism housings (hereinafter referred to as C) disposed on the lower head of the reactor pressure vessel 1.
(Referred to as an RD housing) 5 is inserted from below and fixed.

The details of CRD4 are as shown in FIG.
The hollow piston 20 is accommodated in the piston 4 so as to be able to move up and down, and the control rod 3 and the hollow piston 20 are integrally connected by a connecting member provided at the top of the hollow piston 20.

The CRD 4 performs a relatively slow insertion / removal operation (shim operation) of the control rod 3 for adjusting and maintaining the reactor power at a predetermined state in a normal state, and an emergency stop of the reactor in an emergency. It has a function of performing quick insertion (scram operation) of the control rod 3. Generally, the shim operation is performed by a motor drive, and the scrum operation is performed by a hydraulic drive.

That is, during the shim operation, the electric motor 6 is driven to transmit its rotational movement to the ball screw shaft 8 via the drive shaft 7, and the ball nut 9 screwed to the ball screw shaft 8 is moved up and down. As a result, the control rod 3 supported and connected to the hollow piston 20 is inserted into or pulled out of the core 2.

On the other hand, during the scram operation, high-pressure water is injected into the CRD 4 from an unshown water pressure control unit (HCU) via the insertion pipe 21 and further through the insertion flow passage 23 formed in the flange 22 of the CRD 4. The high pressure water presses and pushes the lower surface of the hollow piston 20 to insert the control rod 3 into the core 2 at a high speed.

In order to know the insertion amount of the control rod 3 during the scram operation, a magnet 25 is mounted on the lower end of the hollow piston 20 and a reed switch is provided outside the CRD housing 5 at regular intervals. 26 is provided to detect the open / closed state of the reed switch sequentially energized by the magnet 25 with the movement of the hollow piston 20 to detect this.

The details of the upper end of the ball screw shaft 8 constitute a journal bearing composed of a bushing 11 and bolts 12 as shown in FIG. 5, and the spindle head 10 and the bushing 11 are used when the CRD 4 performs shim operation. Is a stationary bearing, and the ball screw shaft 8 and the bolt 12 are the rotating shaft. The ball screw shaft 8 and the ball nut 9 have excellent corrosion resistance in high-temperature water, have sufficient abrasion resistance against driving during their life, and have sufficient mechanical strength as a structure for supporting the control rod. Therefore, precipitation hardening stainless steel (for example, JIS SUS630) is used as a material.

[0011]

In the CRD 4 having the above structure, the ball screw shaft 8 and the ball nut 9 are slightly affected by the magnet provided at the lower end of the hollow piston 20 because of the use of a weak magnetic material. It becomes magnetized. In particular, the tendency is remarkable at both ends of the ball screw shaft. For this reason, foreign matter such as cladding in the furnace is likely to be adsorbed, and the function of the journal bearing at the upper end of the ball screw shaft may be impaired.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. Even when the ball screw shaft and the ball nut are magnetized, foreign matter easily adheres to the ball screw shaft, the upper end of the ball screw shaft can be used. An object of the present invention is to provide a control rod drive mechanism in which the function of the journal bearing provided in the control rod is not hindered. [Configuration of the Invention]

[0013]

According to the present invention, there is provided a ball screw shaft rotatably inserted and installed in a control rod drive mechanism housing which is penetrated through a reactor pressure vessel and fixed thereto. a ball nut to move screwed to the screw shaft along the ball screw shaft even I Do and the rotation of the ball screw shaft is connected to control rod tip, also in the magnet part is attached the ball a hollow piston which is No設the nut, and a pressing means for pressing <br/> pressure when required space of the control rod drive mechanism housing, which is formed on the bottom side of the hollow piston, said control rod drive mechanism housing is the additionally provided on the outside, and a position detector that is sensitive to the magnetic force upon approach of the magnet, the upper end of the ball screw shaft
Spindle head is rotatable via journal bearing
A control rod drive mechanism provided above the ball screw shaft.
The ball facing the sliding surface of the journal bearing at the end
Made of non-magnetic molybdenum-nickel based alloy on the screw shaft side
The ball screw shaft is attached
It is made of stainless steel .

[0014]

[Function] Even if the ball screw shaft is magnetized by the position detecting magnet provided at the lower end of the hollow piston and foreign matter easily adheres to the ball screw shaft, the spacer formed of a non-magnetic material can be magnetized. And does not adsorb foreign matter.
Therefore, foreign matter does not collect on the journal bearing portion, and the function of the journal bearing is not hindered by the foreign matter.

[0015]

One embodiment of EXAMPLES control rod drive mechanism according to the present invention Nitsu
This will be described with reference to FIGS. FIG. 1 is a longitudinal sectional view of a CRD to which the present embodiment is applied.
In the RD 4, the CRD housing 5 penetrates the lower head plate of the reactor pressure vessel 1 and is mounted in a watertight manner by welding. At the bottom of the CRD housing 5, a motor 6 coaxially connected with a synchro position detector 24 is mounted concentrically therewith, and the motor 6 is extended inside the CRD housing 5.
The ball screw shaft 8 is connected to the drive shaft 7. A ball nut 9 is screwed onto the ball screw shaft 8, and the rotation of the ball nut 9 with respect to the CRD housing 5 is restricted. The ball screw shaft 8 and the ball nut 9 are stainless steel.
It is formed of less steel.

A hollow piston 20 is mounted on the ball nut 9 so as to surround the ball screw shaft 8. A magnet 25 is buried in the base of the hollow piston 20, and its outer periphery is located inside the CRD housing 5. Watertight. The top of the hollow piston 20 can be connected to the lower end of a control rod (not shown).

In the flange 22 provided near the lower end of the CRD housing 5, the CRD on the bottom side of the ball nut 9 is provided.
An insertion channel 23 communicating with a space in the housing 5 is provided,
An insertion pipe 21 is connected to the insertion channel 23, and a water pressure control unit (HC) installed outside the CRD housing 5 is installed.
U) 30. A lower end is supported by a motor bracket 27 that supports the electric motor 6 at the bottom of the CRD housing 5, and a position detector 26 is provided so as to pass through the flange 22 and extend along the outer surface of the CRD housing 5.

FIG. 2 is an enlarged longitudinal sectional view showing the upper end of the ball screw shaft 8 according to the present embodiment in detail. That is, in FIG. 2, the spindle head 10 is fitted into three grooves provided in the axial direction on the inner surface of the hollow piston 20 shown in FIG.
Is installed. Spindle head 10 is bushing
11 and fixed by hexagon bolts 15.

Further, the bushing 11 and the bolt 12 constitute a journal bearing. At the upper end of the ball screw shaft 8, an annular spacer 17 having a female screw on the inner surface formed of a nonmagnetic material is provided. The spacer 17 is fixed by screwing the bolt 12 at the same time, and the pin 16 is attached through the ball screw shaft 8 and the bolt 12 to prevent the bolt 12 from loosening.

The spacer 17 may be any non-magnetic material which is excellent in corrosion resistance and wear resistance in high-temperature water.
Table 1 shows the composition of the Ni alloy as an example. The length of the spacer 17 is set to a predetermined length in order to prevent foreign matter from adhering to the journal bearing portion due to the slightly magnetized ball screw shaft 8. Further, the spacer 17 is not directly provided only at the upper end of the ball screw shaft 8,
Can be interposed between the tip of the bearing and the journal bearing.

[0021]

[Table 1]

Next, the operation of the above-described device will be described.
When a control rod for normal power output control is inserted into and pulled out of the reactor core (shim operation), the electric motor 6 is used.
By rotating the ball screw shaft 8 forward or backward, the rotation of the ball screw shaft 8 is converted into a vertical movement of the ball nut 9, and the hollow piston 20 mounted on the ball nut 9 also follows the control rod connected therewith to form a control rod. Insert and withdraw into the core. The amount of movement of the control rod, that is, the amount of rotation of the ball nut 9 is detected by a synchro position detector 24 connected to the electric motor 6.

When the operation of rapidly inserting the control rod into the reactor core (scram operation) is performed, the water pressure control unit (H
High pressure water is introduced into the CRD housing 5 from the CU) 30 through the insertion pipe 21. The pressure of the high-pressure water acts on the ball nut 9 and does not move it, but acts on the bottom surface of the hollow piston 20 placed on the ball nut 9 to cause the hollow piston 20 to move.
Is separated from the ball nut 9 and pushed up, so that the control rod is inserted into the core and the scram operation is performed irrespective of the drive system by the ball screw shaft 8 or the like.

In this scram operation, the drive system using the ball screw shaft 8 and the like does not operate, so that the position of the control rod cannot be detected by the synchro position detector 24.
Therefore, instead of this, the magnetic field of the magnet 25 which moves with the insertion of the control rod acts, and the arrangement position of the reed switches of the position detector 26 which sequentially opens and closes, or the opening and closing of the adjacent reed switches. Intervals and the like are detected, and the position or moving speed of the control rod is determined from these detection results.

Further, during the shim operation, the bushing 11 and the bolt 12 constitute a journal bearing. That is, during the shim operation of the CRD, the spindle head 10 and the bushing 11 serve as bearings, and the ball screw shaft 8 and the bolt 12 serve as rotating shafts.

In such a configuration, the ball screw is provided at the lower end of the hollow piston 20 for the position detecting magnet 25.
The spacer 17 is made of a non-magnetic material that can be said to be perfect even when the magnets are magnetized and foreign matter easily adheres to the ball screw shaft 8, so that the spacers 17 do not magnetize and the foreign matter is attracted to the spacer 17. It will not be done. Therefore, the function of the journal bearing provided on the upper portion of the spacer 17 is not hindered by the entry of foreign matter.

In this embodiment, the material of the spacer 17 is a Ni-based alloy shown in Table 1. However, any material can be used as long as it is a non-magnetic material having excellent corrosion resistance and abrasion resistance in high-temperature water. Stellite 5, Stellite 25, Inconel 718, Hastelloy alloy, Nitronic 60, non-magnetic material stainless steel, hard chromium plating or material hardened by colmonoy can be used.

[0028]

According to the present invention, even if the ball screw shaft is magnetized and foreign matter easily adheres, the function of the ball screw shaft during shim operation is not impaired, and reliability is secured. Therefore, it is possible to provide a control rod drive mechanism capable of improving the reliability of the entire reactor.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a control rod driving mechanism to which an embodiment of the present invention is applied.

FIG. 2 is an enlarged partial sectional view showing an upper end portion of the ball screw shaft of FIG. 1;

FIG. 3 is a cross-sectional view illustrating a main part of a nuclear reactor related to a control rod drive mechanism.

FIG. 4 is a sectional view schematically showing a conventional control rod driving mechanism.

FIG. 5 is an enlarged partial cross-sectional view showing an upper end portion of the ball screw shaft of FIG. 4;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Reactor pressure vessel, 3 ... Control rod, 5 ... Control rod drive mechanism housing, 8 ... Ball screw shaft, 9 ... Ball nut, 11
... bushing, 12 ... bolt, 17 ... spacer, 20 ... hollow piston, 25 ... magnet, 26 ... position detector, 30 ... water pressure control unit.

Claims (1)

(57) [Claims] 1. A ball screw shaft which is rotatably inserted into a control rod drive mechanism housing penetrating a reactor pressure vessel and fixed thereto, and a ball screw shaft screwed to the ball screw shaft. a ball nut which moves along the ball screw shaft I and Do to rotation of coupled control rod tip and a hollow piston which is No設to the ball nut magnet part is attached and pressurizing means for pressurizing the space of the control rod drive mechanism housing, which is formed on the bottom side of the hollow piston <br/> when required, it is additionally provided on the outside of the control rod drive mechanism housing, the magnet and a position detector that is sensitive to the magnetic force at the time of approach of the ball
Rotatable via a journal bearing at the upper end of the screw shaft
In a control rod drive mechanism with a spindle head,
And the sliding surface of the journal bearing at the upper end of the ball screw shaft.
A non-magnetic molybdenum-d
Attach an annular spacer formed of a nickel-based alloy, and
A control rod drive mechanism characterized in that the ball screw shaft is made of stainless steel .